The invention relates to a resistor which comprises a substrate having two connections which are electrically interconnected via a resistance path, said resistance path comprising a first path portion having a positive TCR (temperature coefficient of resistance) and a second portion having a negative TCR.
Resistors of the type mentioned above are referred to as "precision resistors". They must have an accurately adjustable and readily reproducible resistance value. The TCR of such a precision resistor must also be accurately adjustable and readily reproducible. The deviation of the resistance value is preferably below 0.1%, and the deviation of the intended TCR is preferably below 1 ppm/.degree. C. The TCR of a resistor is to be understood to mean the relative change of the resistance as a function of temperature. The TCR is generally given in ppm/.degree. C. In general, the aim is to manufacture precision resistors whose TCR is substantially zero.
A resistor of the type mentioned in the opening paragraph is known per se. For example, in U.S. Pat. No. 4,907,341, a description is given of a compound resistor of this construction. In this case, the resistance paths of the individual resistors are composed of a first path portion, which is referred to as "adjustable portion" and a second path portion which is referred to as "resistance portion". The adjustable portion consists of an electrically well-conducting material having a relatively high, positive TCR. The resistance portion consists of a customary resistance material having a relatively low, negative TCR. In a typical exemplary embodiment of said Patent Specification, NiCr having a TCR in the range from -30 to 0 ppm/.degree. C. is used as the material for the resistance portion, while Ni having a TCR of +5000 ppm/.degree. C. is used as the material for the adjustable portion. The difference in resistance between both portions amounts to a factor of 1000.
In the manufacture of the known type of resistors the resistance value and the TCR are adjusted as follows. First, the resistance of the individual resistors is brought to the desired value by etching away the first path portion of resistance material. Subsequently, the TCR of the individual resistors is reduced to a value close to 0 ppm/.degree. C. by subjecting the second path portion of the conductive material to an operation which is also referred to as "trimming".
The manufacture of the known resistor has several disadvantages. For example, it has been found in practice that the TCR of untrimmed resistors can vary quite substantially as a result of small variations in the composition of the resistance material. As a result, a substantial portion of the mass-produced resistors already have a positive TCR before they can be subjected to the trimming process. Under these conditions, the TCR of these resistors can no longer be reduced to a value close to zero. The reason for this being that trimming of the adjustable portion leads to a further increase of the (positive) TCR. Due to this disadvantage, a considerable number of the resistors manufactured must be rejected. Moreover, adjusting of the TCR causes the accurately adjusted resistance value to change.